TMT4320 - Nanomaterialer

Fakta høst 2008 Foreleser: Fride Vullum Stud-ass: ? Vurderingsform: Skriftlig eksamen Eksamensdato: 18. desember

Øvingsopplegg høst 2008 Antall godkjente: 6/12 Innleveringssted: Utenfor R7 Frist: Tirsdager 16:00 (?)

Emnet skal gi en innføring i grunnleggende kjemisk prinsipper for å lage nanomaterialer. Stikkord: "Self-assembled" monolag (SAM) og hvordan disse kan formes ved myk litografi og "dip pen" nanolitografi, syntese av tredimensjonale multilag strukturer. Tynne filmer ved kjemisk gassfase deponering. Syntese av nanopartikler, nanostaver, nanorør og nanoledninger. Våtkjemiske syntese av oksidbaserte nanomaterialer. "Self-asembly" av kolloidale mikrokuler til fotoniske krystaller, porøse nanomaterialer, blokk-kopolymere som nanomaterialer. "Self assembly" av store byggeblokker til funksjonelle anordninger.

Et lite kompendium i faget

Her vil det etterhvert vokse fram et lite kompendium i faget. Dette følger i utgangspunktet pensumlista som gjelder for høsten 2008.

Chapter 2: Soft Lithography

Self-assembled monolayers (SAMs)

• The typical example of a SAM is a layer of alkanethiols on a gold substrate.
• The S-H bond is cleaved on the gold surface and an Au-S covalent bond is formed.
• The alkanethiols are tilted off-axis from the normal. The angle depends on the gold surface. (30 °C for a {111} surface).
• The end group on the alkanethiols can be tailored to achieve different monolayer properties.

PDMS stamp

• PDMS = PolyDiMethylSiloxane
• A master (casting) of the stamp, with the desired pattern, is made with lithography. The master is silanized and made hydrophobic so removing the stamp becomes easier.
• Liquid PDMS is then poured into the master, after which it is cured and a finished PDMS stamp is removed from the master.
• The critical dimensions of the pattern are limited by the lithography techniques used, and for photolithography the wavelengths of the light used to expose the photoresist limits the dimensions. Typical CDs given are, for lateral dimensions within the range of 500nm-200µm, and for the height of patterns 200nm-20µm.
• The PDMS stamp can be dipped in alkanethiol solutions (or solutions of other molecules, collectively known as "chemical ink") and be stamped onto surfaces
• PDMS stamps work on both planar and curved surfaces
• For the stamp to properly print a pattern onto a surface, the molecules need to adhere to the stamp from the solution, but needs to adhere more strongly to the surface to be printed on.

Hydrophilic / Hydrophobic stamps

• The endgroup/terminal group on the alkanethiols (or other molecules used) determine the properties of the monolayer
• By introducing a wetability gradient or abrupt changes in wetability, different effects can be obtained
  • Square drops, by having checkerboard square patterns of hydrophobic/hydrophilic monolayers, and condensating a vapor onto the surface
  • Drops "running uphill" by having wetability gradients

Printing thin films

• As long as the adhesion between the chemical ink and the substrate is stronger than the adhesion between the ink and the stamp, printing thin films is no problem
• Metal thin films can be evaporated onto the stamp (evaporation gives homogenous and directional coatings, not covering the side walls on the stamp) and printed onto a substrate that has been primed with a SAM with exposed thiol groups (adheres strongly to the metal layer)
• This is a very gentle technique for metal film depositing, good for making contacts on fragile layers. Also good for making 3D stuctures by printing multiple layers.

Electrically conducting SAMS

• Electronic devices will always need to make electrical contact with SAMs
• Other, less gentle methods of metal deposition than printing with PDMS stamps (sputtering, CVD, etc) can cause the metal layer to penetrate the SAM
• Morale: Use stamps to deposit metals on SAMs!

Patterning by photocatalysis

• Photocatalysis is used to remove parts of a SAM (making patterns)
• Titania can photocatalytically decompose organic molecules.
• A quartz slide patterned with titanium dioxide in the required pattern is pressed against a wafer with the SAM on it.
• The assembly is exposed to UV irradiation, triggering the degeneration of the (organic) SAM

Kapittel 3: Building layer-by-layer

Gøran er på saken.

Electrostatic superlattices

• Lbl multilayer films formed by alternate immersion in suspensions of opposite charges
• A primer layer with a charge adheres to the substrate. The substrate is then dipped in a solution of polyelectrolytes of opposite charge from the primer layer. Repeated with opposite charges.
• As the amount and identity of constituents of each layer can be controlled, a composition gradient can easily be constructed throughout the structure.
• Any species bearing multiple ionic charges can be layered.

Some applications

• Electrochromics layers (change color when a potential is applied), used in "smart windows" for instance
• Construction of cantilevers for AFMs and similar equipment, using photolithography and lbl

Analysis, measuring film thickness

• Optical spectroscopy: If the substrate is transparent, and the film absorbs light at a certain wavelength, the film thickness can be found by monitoring the optical absorption as a function of number of layers. A dye can be introduced to ensure absorption. Easy to perform but hard to interpret - must know the observation area and extinction coefficient of the absorbing group.
• Ellipsometry: Film is probed by polarized light, and change in polarization in the reflected light is measured. This can be used to find the refractive index, thickness, roughness and orientation of a thin film. Ellipsometry works with films much thinner than the wavelength of light - down to atomic layers.
• Quartz crystal microbalance (QCM): Quartz (piezoelectric) in an alternating electric field contracts/expands with a characteristic oscillation frequency. When mass is added to QCM the frequency decreases. This allows real-time thickness measurements. Works well for hard materials like metals and ceramics, but not for viscoelastic materials.
• Direct techniques: Label each layer with heavy metal atoms and image by TEM. Alternately, image cross section by TEM.

Non-electrostatic lbl assembly

• Lbl doesn't need electrostatic bridges - can use hydrogen bonding, ligand-receptor interactions or even covalent bonds.
• Example: DNA (adenine-thymine and guanine-cytosine bridges)
• Hydrogen bonds can be broken again by changing the pH, or can be strengthened by UV irradiation

Low-pressure layers

• Molecular beam epitaxy (MBE): Performed in a vacuum, sources of constituents (elemental) are heated, and a thin film alloyed from the constituents is deposited. The result is a homogeneous crystal. The substrate should have a similar lattice constant to that of the layer deposited.
• Chemical vapor deposition (CVD): Volatile precursors are introduced in gas phase in a low-pressure reactor chamber. Argon gas is used to dilute the precursor gas to achieve optimal pressure and concentration. The substrate is heated, and the precursor decomposes at the surface.

Lbl self-limiting reactions

• Atomic layer deposition: Similar to CVD, but usually carried out in solution.
• Iterative saturating reactions.

Kapittel 4: Nanocontact printing and writing

Dag H. jobber med kap.4

• Soft lithography and microcontact printing

-Sub 100 nm Soft Lithography: Previous chapters has covered printing on 10.000-100 nm scale. Need for further miniaturization because of demand for more power, efficiency, and density. This can be done by manipulating PDMS stamp, Dip Pen Nanolithography (DPN),Whittling Nanostructures or by Nanoplotters

• • Manipulating PDMS stamp: Manipulating PDMS stamp can be done in various ways, and seven of the basic ideas will now be explained. Illustrating pictures are in the book and in foils. 1) Compress the stamp, mold to get a new stamp with inverse pattern, peel off and repeat. 2) Apply force perpendicular onto stamp when on substrate. The areas in contact with substrate will then increase, and spaces in between gets smaller. 3) Size reduction by reactive spreading some sort of ink when in contact with substrate. The contact time + properties of the ink decide to which degree the ink spreads. 4) Size reduction by extraction of inert filler (just like retracting water from a sponge). 5) Size reduction by swelling the stamp in toluene. 6) Size reduction by stretching stamp so that dimensions get smaller in one axis and larger in another. 7) Size reduction by double-printing. Limitations:Deformation can be a shortcoming if care is not taken with the dimensions of surface relief pattern in the stamp as this can give unwanted deformations. Quality of printed pattern will not be good. Defect-free contact printing is restricted to a certain range of height-to-width ratios. If ratio is outside 0,2-2, the roof of the grooves on stamp will touch the substrate.Too high perpendicular force on stamp has the same effect, but overpressure can also be used to form new patterns such as micron scale discs and rings of ferromagnetic core-shell nanoparticles. Nanoparticles are then transferred to PDMS stamp by Langmuir-Blodgett technique (chapter 6) and then into contact with Au-coated silicon substrate. Low pressurediscs, high pressurerings.

• • Dip pen nanolithography: Alkanethiols can be written on gold substrate with AFM tip. The alkanethiols are delivered to the tip via a water meniscus, and this can be adapted to suit other surface chemistries. The result is 10 nm fine patterns of molecules (biomolecules, polymers etc.) on metals, semiconductors and dielectrica.
    • Sol-gel DPN:patterning of solid-state materials. Nanoscale patterns are written using a metal oxide sol-gel precursor in a solvent carrier. The sol-gel precursors are hydrolyzed to metal oxide by use of atmospheric moisture and water meniscus at the tip-substrate interface. pH, substrate temperature and post treatment can be varied.
    • Enzyme DPN: A scanning microscope tip can be used to place an enzyme on a specific site on a biomolecule with nanometer presicion. This method leads to the possibility of bionanodegradable electronic and optical devices.
    • Electrostatic DPN: Like thin films can be made of charged polyelectrolytes, an AFM tip can "draw" lines or structures of charged polymers with for example specific electrical properties to build nanoscale electronic devices.
    • Electrochemical DPN: The meniscus that forms between surface and tip is used as a nanochemical reactor. Electrochemical deposition can be done by applying voltage between tip and substrate. Ex: making platinum lines can be made by reducing Pt salt at -4 V, and silica lines can be made by oxidation of silicon surface at +10 V.

• • Whittling of nanostructures (section 4.19)
  • Only be able to explain basic principle
    • The spatial extent of SAMs can be reduced by so-called "whittling". Whittling is an electrochemical desorption process where a voltage applied will cause ligands to desorbate. It has been found that the larger the accessibility of a molecule, the lower the desorbation voltage is (fig. 4.22)
• Nanoplotters and nanoblotters
  • What are these and what can they be used for?
    • Nanoplotter: Parallel cantilevers write SAM nanopatterns simultaneously.
    • Nanoblotters: An PDMS inkwell has been created to deliver ink to the nanoplotter cantilever tips (fig. 4.26)
  • Be able to explain basic principles.
• Combinatorial libraries
  • Be able to explain the basic principle and how it is used to find new and improved materials.
    • Combinatorial libraries: DPN can be used to put different materials together in the research of new material composition. With DPN, many different combinations can be made with small material amounts used.

Kapittel 5: Nano-rod, nanotube, nanowire self-assembly

Dag H. skriver på denne også. Flere må legge til ting!!

• Templates for synthesis of nanorods
  • How to make Si and Al2O3 templates
    • Straight pores vs modulated diameter pores.
  • Need to know basic principles behind both synthesis methods. Which parameters determine diameter, ordering, length etc?

• How are these templates used to make nanorods and nanotubes
  • Complete filling gives nanorods – electrodepositionI (also called electroplating) and electroless depositionII. Be able to explain the synthesis route for these two methods.
  • Partial filling gives nanotubes – spontaneous wetting using sol-gel or grow layer-by-layer using CVD or ALD.
  • Modulated composition nanorods.
• Magnetic nanorods (sections 5.7 and 5.8)
  • Explain how they assemble based on the geometry of the magnetic segment.
  • Explain how magnetic nanorods can be used to separate specific molecules from a solution.
• Be able to explain how you can make nanorods with both axial and radial composition profiles. Which methods can be used? Also be able to explain how nanorods with a radial composition profile can be used to make nanotubes.
• Single crystal nanowires
  • Synthesis methods
    • VLS synthesis (section 5.15)
    • SFLS synthesis (section 5.17)
    • Pulsed laser deposition
  • How can you make them branch out?
  • Nanowire quantum size effects (section 5.18)
  • Alignment methods
    • Electric field based alignment
    • Microfluidic approach
    • Langmuir-Blodgett
  • How can you get the nanowires to grow in ordered arrays either parallel or perpendicular to the substrate? (Identical to methods used for carbon nanotubes)
  • Application areas
    • LED – be able to explain briefly how to make a nanowire LED and what the important factors are to make a good quality device.
    • Transistors – be able to explain briefly how you can make a simple transistor and how it can be used as a sensor by exploiting adsorption dependent conductivity.
    • Nanowire UV photodetector (section 5.35)
• Simplifying complex nanowires (section 5.36 and lecture notes)
  • Template method
  • Hydrothermal synthesis
• Electrospinning (sections 5.39, 5.40 and lecture notes)
• Carbon nanotubes (sections 5.41, 5.42, 5.44, 5.45-5.48 and lecture notes)
  • What are carbon nanotubes? Be able to describe the three different structures they can have and how their properties are different.
  • Be able to describe briefly (basic principles) at least two of the three main methods used to synthesize carbon nanotubes
    • Arc discharge
    • Laser ablation
    • CVD
  • How can the different structure nanotubes be separated from each other and from other carbon particles.
  • Be able to say something about their properties
    • Mechanical
    • Electrical
    • Chemical
  • Know some about carbon nanotube chemistry (reactivity on the surface vs the ends etc.)
  • Aligning of carbon nanotubes
    • Evaporation induced self-assembly
    • Patterned hydrophilic SAM on substrate – carbon nanotubes will assemble only on the hydrophilic patches.
    • Alignment by pre-existing patterns
      • Perpendicular to substrate
      • Parallel to substrate
    • AC/DC electric fields
  • Applications of carbon nanotubes
    • Sensors
    • Strengthening of materials (composites)
    • Added to materials to improve conductivity

Kapittel 6: Nanocluster Self-Assembly

Anne Kirsti jobber med saken

Capped nanoclusters

A capped nanocluster is a nanometer scale particle with well-defined positions of the constituent atoms. They nucleate from atoms and enter a size range where they behave electronically as molecular nanoclusters. As the number of atoms increases further, they cross over into the nanoscale size domain where quantum size effects dominate, they become quantum dots. A capped nanocluster has a monolayer of a capping ligand on the surface, which can be a polymer or an alkane thiol (if the surface is silver or gold) or some other molecule with an end group that will bind to the surface of the nanocluster. The capping molecules will prevent further growth of the nanocluster. Capping groups serve multiple purposes:

• Change solubility properties
• Enable size-selective crystallization
• Surface functionalization
• Protect nanoclusters from luminescence or charge-carrier quenching

General principles for synthesis of capped nanoclusters (arrested nucleation and growth)

One general synthesis method is the arrested nucleation and growth synthesis. The basic idea is to rapidly create a large number of nucleated seeds (of desired materials) and then allow these to grow at the same rate below supersaturation conditions. This method can be described by the following steps:

• Desired precursors are added to a solution containing a proper capping agent, which is held at an intermediate temperature (200-400 °C depending on the materials. Temperature needs to be high enough to overcome the activation energy for the reaction.).
• Precursors need to be added at an amount that is over the saturation point for the materials in that specific solution.
• Materials will rapidly nucleate (precipitate) and start growing. Once the first molecules have reacted and created a small seed, the energy required for further growth is smaller than the initial activation energy. The nucleated seed can therefore continue to grow below the saturation concentration for the precursor materials.
• Once the nanoclusters reach a certain size range, which may vary from one material to the other, the capping agents will adsorb on the surface of the nanoclusters and prevent further growth. The nanoclusters that are formed will not all have the same diameter, but a range of different diameter clusters will be formed. This can be due to for example concentration gradients in the reactor or reaction medium.

Minimize size dispersity by confining the reaction space

The size of the capped nanoclusters can be controlled by growing them in nanowells made by the methode in figure x. The nanowells are obtained by patterning a silicon wafer with a layer of well-ordered microspheres. By pressing the microspheres against a the wafer and at the same time melt the surface of the wafer with a pulsed laser molten silicon will flow into the voids between the spheres. The size of the nanowells depend on the size of the spheres, the energy density of the laser pulse and applied mechanical pressure, while the size of the crystals depend on the well volume and concentration of the reactants. The crystals can be removed by ultrasound. The downside of the approach is that the amount of nanocrystals obtained will be quiet small.

Tuning properties through physical dimensions rather than chemical composition (QSE)

When electrons are confined in space the size invariant continuum of electronic states of bulk matter transformes into size dependent discrete electronic states in a quantum dot. At the 1-5 nm length scale, which is the CdSe nanocluster size range, the parent continuous electron bands of the bulk semiconductor becomes discrete. The nanoclusters then belong to the quantum size regime, and the properties begin to scale in a predictable fashion with size. By looking at the Schrödinger wave equation it can be seen that there is a blue quantum size effect shift in the energy of the first exciton band or band gap that scales with the reciprocal of the square of the radius of the nanocluster. The wavelengths absorbed change, and the colors of the nanoclusters can be alterd from yellow to red, by changing the physical size of the clusters

How can different phases occur for smaller size particles?

Similar to temperature and pressure, phase transformations in bulk materials are dependent on size. Phase transitions that are prohibited or slowed down by activation energies in the bulk can occur much more readily in nanocrystals of same material. Because of the small size of the crystal the influence of bulk and surface-free energies are different from in a bulk matter. Phase transformations show a distinct dependence on nanocrystal size. It can be shown that phase of nanoclusters can change just by exposing them to a different chemical environment at room temperature.

Makeing nanoclusters water soluble

Why? Water is cheap, widely available and use of it avoides the disposal o organic solvents, which can be quiet harmful for the environment. (Green chemistry). You can use the same principles as for the SAM surface chemistry. A hydrophilic SAM is made by choosing a hydrophilic group such as a carboxylate, ammonium or oligo ethylene glycol. In the case of a gold nanocluster, a thiol with a terminal carboxyl group gives an ionized, water loving carboxylate when in aqueous solution. Hydrophobic nanoclusters can be wrapped by amphiphilic polyers. The polymer coating is stabilized by partially cross linking the anhydride gropuos with bis(6-aminohexyl)amine. Can also coat with silica. Often, the resulting crystals bear a surface charge, which allows their use in electrostatic layer-by-layer deposition.

Separation of nanoclusters by size using using a non-solvent and centrifugation

Nanoclusters can be dissolved in toluene and by gradually adding a non-solvent (e.g. acetone) the nanoclusters will precipitate. The largest clusters precipitate first. Every time a bit of acetone is added the solution is centrifuged and the precipitate collected. The result is highly monodisperse nanoclusters collected in each fraction.

Superlattice

A superlattice is a material with periodically alternating layers of several substances. Such structures possess periodicity both on the scale of each layer's crystal lattice and on the scale of the alternating layers.

Assembling of superlattices

A superlattice can be assembled by means of these techniques:

• Tri-layer solvent diffusion crystallization - Three immiscible solvents are arranged to form separate layers in a test tube. Bottom layer →capped CdSe nanoclusters dissolved in toluene. Middle layer →buffer layer of 2-propanol selected for poor solvent properties wrt the nanoclusters. Top layer →non-solvent for the nanoclusters such as methanol. The process involves slow diffusion of the nanoclusters from the toluene bottom layer and the methanol from the top layer into the buffer layer. The change in solvent properties causes a slow and controlled nucleation and growth of capped CdSe nanocluster crystals.
• Sedimentation –
• Evaporation induced self-assembly – Strong capillary forces in an evaporating water meniscus drives the nanocomponents into close-packing.
• Langmuir-Blodgett – A dilute monolayer of capped silver nanoclusters is spread on an air-water interface. Using Langmuir – Blodgett “equipment”, this monolayer can gradually be compressed until a compact monolayer is formed.

Gjenstår

• Why do we want to make superlattices? (change of properties, properties of superlattice does not necessarily equal the sum of the properties of the individual constituents)How can capping agents (different type and length) affect the properties of a superstructure? (section 6.15)Alloying core-shell nanoclusters

• Nanocluster-polymer composites
  • What is it?
  • How can it be used for down-conversion of light?
• Be able to give one or two examples of how different size nanoclusters labeled with different fluorescent molecules can be used in biology.
• What is a tetrapod and what is the main priciples of the synthesis behind the tetrapod?
  • Using a material that has two common crystal polymorphs where growth of one over the other can be controlled by synthesis temperature.
  • Use of a long chain molecule which selectively binds to specific facets of the structure and hinders growth in those directions. This confines the growth of the material to one spatial dimension.
• Photochromic metal nanoclusters (section 6.31)
  • Be able to explain what happens to silver nanoclusters embedded in a titania matrix when it is exposed to either UV-light or visible light.
• What is a buckyball and what can it be used for? What special properties does it exhibit? (Do not need to know specific details of synthesis or assembly techniques.)

Kapittel 7: Microspheres – Colors from the Beaker

Marius holder på med denne biffen og håper dem blir medium til rå...

What is a photonic crystal?

• It is a crystal consisting of a material with high dielectric contrast and periodicity at the light scale
• Vullums definition: Natural gratings that diffract light are based on dielectric lattices with periodicity at optical wavelengths. 3D optical diffraction gratings have dielectric lattices that are geometrically complimentary.
• 1D PC (planes) is a crystal which only inhibit light to travel in one direction
• 2D PC (rods) inhibits light to travel in two directions
• 3D PC (spheres) inhibits litght to travel in any direction and has a full photonic band gap (PBG), whilst 1D and 2D only have so called stopgaps

Be able to explain how photonic crystals can be used to confine and guide light by the controlled synthesis of different defects

• Point defects: Holes in a 3D PC can trap light inside the crystal
• Line defects: Many holes which make a line can guide light through a crystal
• Plane defects: A missing plane or a defect in a plane can make photons slip through to the other side.

Be able to explain at least two different methods used to induce defects in a material

• Writing defects:
• Synthesizing planar defects by introducing a dense layer or a layer with spheres of a different size than the surrounding colloidal crystal.

Be able to explain how you can tune the color by changing size of the structure and changing dielectric contrast. What happens if the spheres are embedded in a shrinkable and swellable matrix? How can this be used as a sensor to detect different cations?

What are core-corona, core-shell-corona and multi-shell microspheres, how can you make them and what is the purpose of making these spheres?

Know the differences between one-stage and re-growth synthesis.

• One stage: Reagents are mixed and the microspheres are obtained in solution by a nucleation and growth
• Re-growth: First a sees is produced. The seed is then allowed to grow in several steps. Surface tension controls the shape, where low surface tension gives spherical particles.

Know what the basic principles of self assembly are. Be able to name and explain the following self-assembly techniques for microspheres

• Sedimentation (be able to explain in more detail): Use Stokes equation to make spheres you want to sedimentate and make crystalls by changing the viscosity.
• Electrophoresis
• Hydrodynamic shear
• Spin coating
• Langmuir-Blodgett layer-by-layer (be able to explain in more detail)
• Parallel plate confinement: Force spheres to assemble by placing them between two parallel plates and slowly moving one plate closer to the other. Important with slow movement to prevent defects. This can be done both dry and in fluid.
• Evaporation induced self-assembly, EISA, (be able to explain in more detail): Capilary forces drive the assembly of spheres in a solution as you remove a wetting plate out of the solution. These the need to be dried and this can cause cracking.

What are colloidal aggregates? Need to be able to explain different techniques for manufacturing different shapes of these, such as template confinement, aggregation in homogeneous emulsion, and electrospraying.

Need to know that the basic principle behind optical quality of colloidal crystals is based both on Bragg’s law of diffraction and Snell’s law of reflection. Need to be able to understand and explain how the color of the diffracted light changes with the distance between lattice plains.

Cracking This happens when the thin hydration layers around the crystal spheres dry out. This creates capillary stress and thermal expansion. To prevent cracking you can dry the crystal slowly, use hydrophobic spheres. Methods for preventing this is:

• <math>SiCl_4</math> reacting within the hydration layer to create a <math>SiO_2</math> layer between the spheres. This is called CVD necking.
• Rehydrate to form multiple layers (foil 6.11.)
• Heat treatment before assembly
• Redeisperse and crystallize without volume contractions

Liquid crystal photonic crystal: Neither a liquid nor a crystal, but an intermediate state of matter. Lacks the long range order of the crystalline state and does not exhibit the randomness of the liquid state. How can the colors of such a crystal be altered and what can it be used for?

Reactions that you need to know:

• Reaction of alkane thiolate with gold. Important to know that alkane thiols have a specific affinity for gold (also keep in mind that silver and gold have very similar properties).
• Reaction that occurs when during anodic oxidation of Al to produce porous alumina membranes.
• Reaction that occurs when silica microspheres are formed from Si(OEt)4 and water (section 7.9): <math>Si(OEt)_4 + 2H_2O \rightarrow SiO_2 + 4EtOH</math>

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